This will be the fifth test of the new 2" motor design. The propellant is standard KNSU 65/35 using the granular or prilled form of KNO3 and powdered sugar. The grains were cast in the standard method, using a dense but thin cardboard as an inhibitor/casting liner. Once the grains were cooled I used a wrap of aluminum foil tape over the cardboard for extra protection.
Fuel Weight: 2.28 lbs.
Grains: 3) 2"x5"
Burn Time: 1.817 Seconds
Total Impulse: 291.21 lb sec.
Isp: 127.72
Max PSI: 903.23
Max Thrust: 423.2 Pounds
Well, there it is. The 2" motor performing as expected at last! Chamber pressure peaking at just over 900 psi, a burn time of over 1.8 seconds, and the Isp a respectable 127, making the motor a low K designation. I also now have my load cell amplifier gain adjusted so the thrust recorded properly.
The only problem I had, I forgot to buy new o-rings for the nozzle, so I only used RTV as a seal and I did have a slight amount of blow by between the nozzle and the casing. I expected as much, the nozzle is a bit of a loose fit in the casing compared to my other motors, and it really needs the o-ring in there. While there wasn't much blow by, I'm sure I lost a second or two of Isp, and a few pounds of thrust. Never the less, I'm thrilled that the motor performed as expected. The dense cardboard and the aluminum tape inhibitor obviously worked well. That also means I can use the motor with its full propellant capacity.
Here is the motor at thrust, notice the smoke coming from below the nozzle, that's the blow by I mentioned. Now I need to cast some new grains for the 2" motor's first flight.